RIBOTYPES AND VIRULENCE GENE POLYMORPHISMS SUGGEST 3 DISTINCT LISTERIA-MONOCYTOGENES LINEAGES WITH DIFFERENCES IN PATHOGENIC POTENTIAL

A total of 133 Listeria monocytogenes isolates were characterized by r ibotyping and allelic analysis of the virulence genes hly, actA, and i nlA to uncover linkages between independent phylogenetic and specific virulence markers. PCR-restriction fragment length polymorphisms revea led 8 hly, 11 inlA, and 2 actA alleles. The combination of these virul ence gene alleles and ribotype patterns separated L. monocytogenes int o three distinct lineages. While distinct hly and inlA alleles were ge nerally found to cluster into these three lineages, actA alleles segre gated independently. These three phylogenetic lineages were confirmed when 22 partial actA DNA sequences were analyzed. The clinical history of the L. monocytogenes strains showed evidence for differences in pa thogenic potential among the three lineages. Lineage I contains all st rains isolated during epidemic outbreaks of listeriosis, while no huma n isolates were found in lineage III. Animal isolates were found in al l three lineages. We found evidence that isolates from lineages I and In have a higher plaquing efficiency than lineage II strains in a cell culture assay. Strains from lineage III also seem to form larger plaq ues than strains from lineage II. A distinctive ribotype fragment and unique 16S rRNA gene sequences furthermore suggest that lineage III mi ght represent a L. monocytogenes subspecies. None of the 20 human isol ates available but 11% of our animal isolates were grouped in this lin eage, indicating that strains in this lineage might have reduced virul ence for humans.